1. Field of the Invention
This invention relates to supertwist type liquid crystal displays with phase difference plates.
2. Description of the Prior Art
In general, supertwist type (or abbreviated to `STN`) liquid crystal displays intrinsically show yellow green or blue color due to the birefrengence of their liquid crystal layers, which can be converted to bright and clear black-and-white displays through color correction using an optical compensator. By this color correction, the display quality is upgraded to the level which enables liquid crystal displays to be utilized for OA equipment such as word processors, computers and the like.
As supertwist type liquid crystal displays with color correction, those of a double-layered type are available, in which a coloring made in the first layer acting as a driving liquid crystal cell is color corrected to provide the displays achromatic in the second layer acting as an optical compensating cell. This structure provides clear black-and-white displays which make a significantly upgraded visibility in comparison with a single-layered crystal supertwist type liquid crystal display. However, since two liquid crystal cells are required, this display has problems of being thick in depth and having an increased weight.
To solve these problems, there has been developed a thin and light supertwist type liquid crystal display by utilizing a phase difference plate as an optical compensator which is composed of a uniaxial oriented polymer film made from polycarbonate, polyvinyl alcohol and the like. However, since this phase difference plate is fabricated by extending a polymer film, there arises a difference in optical nature of the film between the direction along which it is extended and the direction perpendicular to the extension. This then will compose another problem that phase difference plate supertwist type liquid crystal displays have greater color changes depending upon viewing angles or elevation angles. That is, they have narrower viewing angles in comparison with double-layered supertwist type liquid crystal displays.
The phase difference plate composed of a uniaxial oriented film makes an optical compensator utilizing its anisotropy. In this arrangement, there is a difference in refractive index (birefrengence) of the film between the direction along which it is extended and the direction perpendicular to the extension. Retardation (.DELTA.n.multidot.d) given by the product of this refractive index (.DELTA.n) and the thickness of the film (d) is a physical quantity defining the phase of light determined when it passes through the film, and differs in the direction along which the film is extended and the direction perpendicular to the extension. For example, in a phase difference plate made from polycarbonate and the like which represents a uniaxial crystal, the retardation increases in the extended direction and decreases in the direction perpendicular to the extension. When combined with a liquid crystal display cell, especially in the direction of the phase difference plate extension, an increased difference arisen between the retardation of the phase difference plate and that of the liquid crystal display cell will cause light passing through the assembly to have a phase difference to result in a colored display. That is, no color compensation is performed and hence narrower viewing angles will result because of lowered contrast of the display.
In addition, for those liquid crystal displays which are used for OA equipment, such as personal computers provided with a mouse or handwriting function requiring fast response, a liquid crystal material with the sum of rise time and decay time 100 ms or less will have its sharpness (.alpha. value) of 1.10 or more and accordingly, an insufficient contrast will result.
FIG. 5 is a figure showing the relationship between the sharpness (.alpha. value) and the response time (ms) of a liquid crystal material. In general, as illustrated, the more the response time increases, the less the sharpness (.alpha. value) becomes, and the contrast falls accordingly. Therefore, the utilization of liquid crystal displays for high speed response in high duty applications was impossible and heretofore they have been used in low duty applications.